The invention relates to an air pressure measuring device for a vehicle tire. The invention further relates to a method for assembling an air pressure measuring device according to the invention air pressure measuring device an inner vehicle wheel rim.
An air pressure measuring device for a vehicle tire and a method for assembling an air pressure measuring device of such kind are known from EP 1 277 601 B1 issued to the Applicant. The known air pressure measuring device essentially comprises three elements: A valve body, which can be inserted in a through-hole in a vehicle wheel rim, a cap nut which can be screwed onto the valve body on the outside of the vehicle wheel rim, and a fastening element in the form of a fixing screw arranged on an end face of the valve body on the side facing the inside of the vehicle wheel rim. The fixing screw is fitted in the valve body by means of a threaded connection, wherein the fixing screw has an outer contour which cooperates with a geometry conformed on a “signal housing” to create a positive lock in such manner that the fixing screw is arranged in the signal housing so as to be displaceable axially but rotationally immobile. The signal housing in turn is connected to the vehicle wheel rim and arranged thereon so as to be rotationally immobile. The cap nut also has a rib formed in the area of its thread. When the cap nut is screwed onto the thread of the valve body, said rib of the cap nut first comes into contact with a surface of the valve body which functions as an end stop for the rib. The effect of this is that initially the cap nut is unable to rotate relative to the valve body any more, but when a torque is applied to the cap nut it rotates about its own longitudinal axis together with the valve body. Due to the rotationally immobile fitting of the fixing screw in the signal housing, the result of this is that the fixing screw is screwed onto the thread of the valve body. As soon as the fastening element has reached its axial end position on the mounting thread of the valve body, a further application of a torque to the cap nut results in the destruction or breakage of the rib, thereby allowing the cap nut to be rotated further on the thread of the valve body. This shortens the axial distance between the end face of the cap nut facing the through-hole in the vehicle wheel rim and the vehicle wheel rim and thus enables the valve body to be tensioned or fixed axially in the through-hole in the vehicle wheel rim. At the same time, the cap nut seals the interior space of the vehicle wheel rim off from the outside, particularly since a sealing element is arranged in the area of the through-hole.
The essential feature is that the air pressure measuring device known from the prior art has threads which cooperate with each other on both the valve body and the cap nut. Particularly in order to avoid corrosion, said components (the valve body and the cap nut) must be made from a corrosion-resistant material and/or must be equipped with a corresponding coating. In the case of the cap nut, this is addressed for example in that the cap nut is made from aluminium and has undergone anodization. It is relative complicated and thus also expensive to manufacture such a cap nut, having a thread as well as the rib described previously and the coating.